172 REFLECTION OF LIGHT 



spot or rarely a minute speck ; the middle plate two 

 parallel specks also minute. In P. squilla each plate 

 has a roundish or squarish spot of yellow, all equal in 

 size, and forming an angular band of spots. The 

 distinctions drawn from the form and dentition of the 

 rostrum, and the gibbous carapace of P. squilla, I need 

 not speak of particularly, as these are sufficiently 

 appreciable in cabinet specimens. 



When viewed with a candle the eyes of the Prawn 

 reflect the light with a glare exactly like that seen in 

 a cat's eyes under similar circumstances. The light, 

 in the case of the Prawn, is seen only when the candle 

 is held between the beholder and the insect, and be- 

 comes brighter and larger the more nearly the flame 

 of the candle is brought to the line which unites the 

 observer's eyes and the object. It might seem as if 

 nothing could possibly be discerned when the flame is 

 absolutely in this line, but it is not so ; both eyes 

 being open, the line of vision of each eye passes on 

 one side of the candle, and we discern the two eyes of 

 the Prawn, like two Jittle globes of fire. 



In the case of the cat the phenomenon is said to be 

 produced by the choroid coat at the bottom of the 

 eye ftapetum lucidumj, reflecting from its polished 

 metal-like surface the entering rays, and converging 

 them by its concavity, as if from a concave mirror. 

 This is simple and perfectly intelligible; but I do 

 not see how a similar effect is produced in the 

 compound eyes of the Crustacea, each of which is 

 composed of a great number of conical lenses with 

 apices inward. What is there in this structure 

 that can represent a concave mirror? Yet no one 



